Method of calibrating a heading reference system

ABSTRACT

A method of calibrating a vehicle&#39;s heading system, such as the attitude heading and reference system of an aircraft or the heading system of a ship, positioned along the Earth&#39;s surface involves obtaining both actual and theoretical readings for the magnetometer of the heading system, and comparing these values to obtain calibration values for the heading system which are then averaged to obtain a universal average gain and offset for the magnetometer. The vehicle may be repositioned, such as to North, South, East, and west magnetic headings, with the procedure repeated at each of these headings, and the calibration values averaged, further increasing the accuracy.

FIELD OF THE INVENTION

The present invention relates to methods of calibrating headingreference systems on vehicles, such as attitude heading referencesystems on aircraft or heading systems on ships, in which themagnetometer in such systems is calibrated using actual and theoreticalreadings at various magnetic headings of the vehicle.

BACKGROUND OF THE INVENTION

Typically, vehicles having a heading system, such as aircraft having anattitude and heading reference system, or AHRS, must be periodicallycalibrated to ensure a minimization of measurement errors due to suchfactors as hard iron disturbances of the aircraft, or other vehicle,such as a ship, having a heading system containing a magnetometer. Mostsuch prior art calibration methods require numerous repositioning of thevehicles, such as the aircraft, such as eight or more differentpositions, to come up with an approximate calibration number. None ofthe prior art methods known to applicant utilize theoretical magneticfield properties of the Earth, such as theoretical values for horizontaland vertical intensity of the magnetic field at the location of thepositioned vehicle, such as the aircraft on the tarmac, for comparisonwith actual magnetometer readings at such a position, thereby increasingthe complexity of the calibration procedure and, potentially, affectingits accuracy. Examples of such prior art methods are disclosed in U.S.Pat. Nos. 7,587,277; 8,061,049; 7,891,103; 7,146,740; and 6,860,023,none of which use the theoretical magnetic components of the Earth'smagnetic field, such as obtained from a web site, to calibrate the AHRSheading. By utilizing the calibration method of the present invention,this calibration procedure is significantly simplified and is animprovement over prior art methods.

SUMMARY OF PARTICULAR EMBODIMENTS OF THE INVENTION

The present invention is a method for calibrating a heading systeminstalled in a vehicle, such as an aircraft AHRS or one installedonboard a ship, and which employs a magnetometer, by using thetheoretical magnetic components of the Earth's magnetic field togetherwith actual magnetometer readings, to calibrate the heading system.

In carrying out the method of the present invention, actual readings areobtained from the magnetometer at one or more headings for the vehicle,such as preferably at four different magnetic headings corresponding toNorth, South, East, and West, and a set of theoretical magnetic fieldproperties of the Earth is also obtained at those locations, such asfrom a web site containing this information. These theoretical valuescomprise values for horizontal and vertical intensity of the magneticfield at each of these locations for the vehicle. A theoretical readingfor the magnetometer at each of these headings is computed and thencompared against the actual magnetometer readings at these samelocations to obtain calibration values for the heading system. Thesecalibration values are then utilized, such as by averaging all of thecalibration values obtained, to provide a universal average gain andoffset for the magnetometer, thus, universally calibrating the headingsystem with respect to measurement errors. This calibration method ispreferably performed with the engine and avionics of the aircraftrunning, or corresponding equipment on the vehicle running. If desired,these calibrations values can be filtered through low pass filters toreduce any effects of noise.

Although as noted above the presently preferred calibration method canbe accomplished at a single position of the vehicle, it has been foundthat the accuracy is preferably enhanced by doing it at the fourdirectional headings of the compass; namely, North, South, East, andWest.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of a process flow diagram inaccordance with the presently preferred method of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Initially referring to FIG. 1, a process flow diagram of the presentlypreferred method of the present invention for calibrating a conventionalvehicle heading system containing one or more magnetometers is shown. Asillustrated by way of example in the process flow diagram of FIG. 1, andas will be explained in greater detail hereinafter, the current verticaland horizontal components of the Earth's magnetic field for theparticular location of the vehicle, such as an aircraft or a ship, areobtained. This is represented by block 10 in FIG. 1. The vehicle ispositioned or aligned to a known magnetic heading, as represented byblock 20 in FIG. 1. The alignment attitude of the magnetometers is thendetermined, as represented by block 30 in FIG. 1. The theoretical valuesof the magnetometers is then calculated based on the vertical andhorizontal components of the Earth's magnetic field, the vehicle headingand attitude, as represented by block 40 in FIG. 1. The theoreticalmagnetometer values are then subtracted from the actual measurements toobtain hard iron calibration values for each magnetometer in the headingsystem, as represented by block 50 in FIG. 1.

Typically, for example, in an attitude heading and reference system, orAHRS, on board an aircraft, the magnetic heading and pitch anglecalibration of the AHRS is a very time consuming operation and is oftenlimited to the geographic vicinity of the airport where the calibrationwas performed. That is not the case with the presently preferred methodof the present invention which may be used, for example, with triaxialmagnetometers aligned to the x, y, and z planes, or with two axismagnetometer arrangements, In either instance, a set of theoreticalmagnetic field properties of the Earth, such as preferably thetheoretical values for horizontal intensity and vertical intensity ofthe magnetic field, at the location of the positioned vehicle isobtained. The information for these theoretical values may preferably beobtained from the web sitehttp:/www.ngdc.noaa.gov/geomagmodels/IGRFWMM.jsp. For example, anaircraft whose conventional AHRS is being calibrated, would preferablybe positioned in a magnetically clean and flat area heading magneticNorth with its engines and avionics running. Preferably the conventionalmagnetic calibration page of the AHRS would be accessed and thetheoretical values obtained from the web site, including the total fieldas well as the horizontal and vertical intensity, would be entered onthe magnetic calibration page and, for example, North Reading would beselected. To improve accuracy, this procedure can preferably be repeatedfor each of the other three normal magnetic headings, East, South, andWest, by first positioning the vehicle to each of these headings,repeating the procedure, and selecting the corresponding Reading forthat heading.

In this regard, it should be noted that the normal magnetometer readingswithout the presence of any hard iron effects are defined by thefollowing equations for each of the four normal headings North, East,South and West:

(a) When heading North:

$\begin{matrix}{\begin{bmatrix}{xMag} & {yMag} & {zMag}\end{bmatrix} = \begin{bmatrix}{HI} & 0 & {VI}\end{bmatrix}} \\{{\quad\begin{bmatrix}{\cos \; \theta} & {\sin \; {\theta sin}\text{?}\varphi} & {\sin \; {\theta cos}\text{?}\varphi} \\0 & {\cos \text{?}\varphi} & {{- \sin}\text{?}\varphi} \\{{- \sin}\; \theta} & {\cos \; {\theta sin}\text{?}\varphi} & {\cos \; {\theta cos}\text{?}\varphi}\end{bmatrix}}} \\{= \left\lbrack {{{HI}\mspace{11mu} \cos \; \theta} - {{VI}\mspace{11mu} \sin \; \theta \mspace{14mu} {HI}\; \sin \; \theta \; \sin \text{?}{\varphi.}}} \right.}\end{matrix}$ ?indicates text missing or illegible when filed

(b) When heading East:

$\begin{matrix}{\begin{bmatrix}{xMag} & {yMag} & {zMag}\end{bmatrix} = \begin{bmatrix}0 & {HI} & {VI}\end{bmatrix}} \\{\begin{bmatrix}{\cos \; \theta} & {\sin \; {\theta sin}\text{?}\varphi} & {\sin \; {\theta cos}\text{?}\varphi} \\0 & {\cos \text{?}\varphi} & {{- \sin}\text{?}\varphi} \\{{- \sin}\; \theta} & {\cos \; {\theta sin}\text{?}\varphi} & {\cos \; {\theta cos}\text{?}\varphi}\end{bmatrix}} \\{= \left\lbrack {{{- {VI}}\mspace{11mu} \sin \; \theta \mspace{14mu} {HI}\; \cos \text{?}\varphi} + {{VI}\mspace{11mu} \cos \; \theta \; \sin}} \right\rbrack}\end{matrix}$ ?indicates text missing or illegible when filed

(c) When heading South:

$\begin{matrix}{\begin{bmatrix}{xMag} & {yMag} & {zMag}\end{bmatrix} = \begin{bmatrix}{- {HI}} & 0 & {VI}\end{bmatrix}} \\{{\quad \begin{bmatrix}{\cos \; \theta} & {\sin \; {\theta sin}\text{?}\varphi} & {\sin \; {\theta cos}\text{?}\varphi} \\0 & {\cos \text{?}\varphi} & {{- \sin}\text{?}\varphi} \\{{- \sin}\; \theta} & {\cos \; {\theta sin}\text{?}\varphi} & {\cos \; {\theta cos}\text{?}\varphi}\end{bmatrix}}} \\{\left\lbrack {{{- {HI}}\mspace{11mu} \cos \; \theta} - {{VI}\mspace{11mu} \sin \; \theta}\mspace{14mu} - {{HI}\; \sin \; \theta \; {si}\text{?}}} \right.}\end{matrix}$ ?indicates text missing or illegible when filed

(d) When heading West:

$\begin{matrix}{\begin{bmatrix}{xMag} & {yMag} & {zMag}\end{bmatrix} = \begin{bmatrix}0 & {- {HI}} & {VI}\end{bmatrix}} \\{{\quad \begin{bmatrix}{\cos \; \theta} & {\sin \; {\theta sin}\text{?}\varphi} & {\sin \; {\theta cos}\text{?}\varphi} \\0 & {\cos \text{?}\varphi} & {{- \sin}\text{?}\varphi} \\{{- \sin}\; \theta} & {\cos \; {\theta sin}\text{?}\varphi} & {\cos \; {\theta cos}\text{?}\varphi}\end{bmatrix}}} \\{= \left\lbrack {{{- {VI}}\; \sin \; \theta}\mspace{14mu} - {{HI}\; \cos \text{?}\varphi} + {{VI}\; \cos \; \theta}} \right.}\end{matrix}$ ?indicates text missing or illegible when filed

where

HI Horizontal intensity of the local magnetic field;

VI Vertical intensity of the local magnetic field;

Θ Aircraft pitch angle;

φ Aircraft bank angle;

ψ Aircraft magnetic heading;

xMag y-axis magnetometer reading;

yMag x-axis magnetometer reading; and

zMag z-axis magnetometer reading.

The deviation from the above values is referred to as the hard ironoffset of the unit being calibrated and, in accordance with thepresently preferred method, will be subtracted from the magnetometerreadings resulting in the following equation for the corrected heading:

$\psi = {- {\tan^{- 1}\left( \frac{Y_{e}}{X_{e}} \right)}}$

Where:

X _(a) =X _(b) cos θ+Y _(b) sin θ sin □φ+Z _(b) sin θ cos □φ

Y _(a) =Y _(b) cos φ−Z _(b) sin □φ

Z _(a) =X _(b) sin θ−Y _(b) cos θ sin □φ−Z _(b) cos θ cos □φ

Thus, summarizing the above presently preferred method for calibrating aheading system installed in a vehicle, actual readings are obtained fromone or more magnetometers at one or more headings for the vehicle, suchas preferably at the four different magnetic headings corresponding toNorth, South, East, and West, and a set of theoretical magnetic fieldproperties of the Earth is also obtained at those same locations, suchas from a web site containing this information. These theoretical valuesinclude values for the horizontal and vertical intensity of the magneticfield at each of these locations for the vehicle as well as the totalfield. A theoretical reading for the magnetometer at each of theseheadings is computed and then compared against the actual magnetometerreadings at these same locations to obtain calibration values for theheading system. These calibration values are then utilized, such as byaveraging all of the calibration values obtained, to provide a universalaverage gain and offset for the magnetometer, thus, universallycalibrating the heading system with respect to measurement errors. Thiscalibration method is preferably performed with the engine and avionicsof the aircraft running, or corresponding equipment on the vehiclerunning. If desired, these calibration values can be filtered throughlow pass filters to reduce any effects of noise.

Although the presently preferred calibration method can be accomplishedat a single position of the vehicle, it has been found that the accuracyis preferably enhanced by doing it at the four normal directionalheadings of the compass; namely, North, South, East, and West.

What is claimed is:
 1. A method for calibrating an aircraft attitude andreference heading system for an aircraft while positioned on the groundalong the Earth's surface, said heading system having an associatedattitude for said positioned aircraft and comprising a magnetometer,said method comprising the steps of: obtaining an actual reading fromsaid magnetometer at a selected magnetic heading for said positionedaircraft; obtaining a set of theoretical magnetic field properties ofthe Earth at the location of said positioned aircraft, said obtainedtheoretical magnetic field properties comprising theoretical values fora horizontal intensity and a vertical intensity of said magnetic fieldat said location of said positioned aircraft; computing a theoreticalreading for said magnetometer at said selected magnetic heading of saidpositioned aircraft based on at least said magnetic heading, saidassociated attitude for said positioned aircraft, and said obtainedtheoretical values for said magnetic field at said location of saidpositioned aircraft; comparing said obtained actual magnetometer readingfor said positioned aircraft at said selected magnetic heading with saidobtained theoretical magnetometer reading at said selected magneticheading for providing a calibration value for said heading system atsaid selected magnetic heading for said positioned aircraft; andutilizing said provided calibration value at said magnetic heading ofsaid aircraft for providing a universal average gain and offset for saidmagnetometer; whereby said heading system is universally calibrated withrespect to measurement errors.
 2. A heading system calibration method inaccordance with claim 1 wherein said aircraft is selectively positionedto at least four different magnetic headings for said aircraft, saidmethod further comprising repeating said obtaining, computing, andcomparing steps at each of said four different selected magneticheadings for said aircraft for obtaining calibration values for saidheading system at each of said four different selected magnetic headingsfor said aircraft, said utilizing step further comprising averaging allof said four provided calibration values for providing said universalaverage gain and offset for said magnetometer.
 3. A heading systemcalibration method in accordance with claim 2 wherein said fourdifferent selected magnetic headings for said aircraft comprise theNorth, South, East, and West magnetic headings for said aircraft duringsaid calibration.
 4. A heading system calibration method in accordancewith claim 1 further comprising repositioning said aircraft to adifferent selected magnetic heading, repeating each of said obtaining,computing, and comparing steps at said different selected magneticheading for said repositioned aircraft for providing another calibrationvalue for said heading system at said different selected magneticheading for said repositioned aircraft, said different selected magneticheadings for said aircraft being selected from the group consisting ofNorth, South, East, and West magnetic headings for said aircraft duringsaid calibration, said utilizing step further comprising averaging saidprovided calibration values at said magnetic headings of said positionedaircraft for providing said universal average gain and offset for saidmagnetometer.
 5. A heading system calibration method in accordance withclaim 1 wherein said magnetometer comprises a triaxial magnetometer. 6.A heading system calibration method in accordance with claim 5 whereinsaid aircraft is selectively positioned to at least four differentmagnetic headings for said aircraft, said method further comprisingrepeating said obtaining, computing, and comparing steps at each of saidfour different selected magnetic headings for said aircraft forobtaining calibration values for said heading system at each of saidfour different selected magnetic headings for said aircraft, saidutilizing step further comprising averaging all of said four providedcalibration values for providing said universal average gain and offsetfor said magnetometer.
 7. A heading system calibration method inaccordance with claim 6 wherein said four different selected magneticheadings for said aircraft comprise the North, South, East, and Westmagnetic headings for said aircraft during said calibration.
 8. Aheading system calibration method in accordance with claim 5 furthercomprising repositioning said aircraft to a different selected magneticheading, repeating each of said obtaining, computing, and comparingsteps at said different selected magnetic heading for said repositionedaircraft for providing another calibration value for said heading systemat said different selected magnetic heading for said repositionedaircraft, said different selected magnetic headings for said aircraftbeing selected from the group consisting of North, South, East, and Westmagnetic headings for said aircraft during said calibration, saidutilizing step further comprising averaging said provided calibrationvalues at said magnetic headings of said positioned aircraft forproviding said universal average gain and offset for said magnetometer.9. A heading system calibration method in accordance with claim 1wherein said step of obtaining said set of theoretical magnetic fieldproperties further comprises the step of obtaining said set from the website http://www.ngdc.noaa.gov/geomagmodels/IGRFNMM.jsp.
 10. A headingsystem calibration method in accordance with claim 9 wherein saidaircraft is selectively positioned to at least four different magneticheadings for said aircraft, said method further comprising repeatingsaid obtaining, computing, and comparing steps at each of said fourdifferent selected magnetic headings for said aircraft for obtainingcalibration values for said heading system at each of said fourdifferent selected magnetic headings for said aircraft, said utilizingstep further comprising averaging all of said four provided calibrationvalues for providing said universal average gain and offset for saidmagnetometer.
 11. A heading system calibration method in accordance withclaim 10 wherein said four different selected magnetic headings for saidaircraft comprise the North, South, East, and West magnetic headings forsaid aircraft during said calibration.
 12. A heading system calibrationmethod in accordance with claim 9 further comprising repositioning saidaircraft to a different selected magnetic heading, repeating each ofsaid obtaining, computing, and comparing steps at said differentselected magnetic heading for said repositioned aircraft for providinganother calibration value for said heading system at said differentselected magnetic heading for said repositioned aircraft, said differentselected magnetic headings for said aircraft being selected from thegroup consisting of North, South, East, and West magnetic headings forsaid aircraft during said calibration, said utilizing step furthercomprising averaging said provided calibration values at said magneticheadings of said positioned aircraft for providing said universalaverage gain and offset for said magnetometer.
 13. A heading systemcalibration method in accordance with claim 9 wherein said magnetometercomprises a triaxial magnetometer.
 14. A heading system calibrationmethod in accordance with claim 1 wherein said aircraft has anassociated engine and avionics, said calibration method being performedwith said aircraft engine and avionics running.
 15. A heading systemcalibration method in accordance with claim 14 wherein said aircraft isselectively positioned to at least four different magnetic headings forsaid aircraft, said method further comprising repeating said obtaining,computing, and comparing steps at each of said four different selectedmagnetic headings for said aircraft for obtaining calibration values forsaid heading system at each of said four different selected magneticheadings for said aircraft, said utilizing step further comprisingaveraging all of said four provided calibration values for providingsaid universal average gain and offset for said magnetometer.
 16. Aheading system calibration method in accordance with claim 14 furthercomprising repositioning said aircraft to a different selected magneticheading, repeating each of said obtaining, computing, and comparingsteps at said different selected magnetic heading for said repositionedaircraft for providing another calibration value for said heading systemat said different selected magnetic heading for said repositionedaircraft, said different selected magnetic headings for said aircraftbeing selected from the group consisting of North, South, East, and Westmagnetic headings for said aircraft during said calibration, saidutilizing step further comprising averaging said provided calibrationvalues at said magnetic headings of said positioned aircraft forproviding said universal average gain and offset for said magnetometer.17. A heading system calibration method in accordance with claim 5wherein said four different selected magnetic headings for said aircraftcomprise the North, South, East, and West magnetic headings for saidaircraft during said calibration.
 18. A heading system calibrationmethod in accordance with claim 1 wherein said magnetometer comprises atriaxial magnetometer and the equation for the aircraft magnetic headingis $\psi = {- {\tan^{- 1}\left( \frac{Y_{e}}{X_{e}} \right)}}$ Where:X _(a) =X _(b) cos θ+Y _(b) sin θ sin □φ+Z _(b) sin θ cos □φY _(a) =Y _(b) cos φ−Z _(b) sin □φZ _(a) =X _(b) sin θ−Y _(b) cos θ sin □φ−Z _(b) cos θ cos □φ
 19. Amethod for calibrating a heading system for a vehicle while positionedalong the Earth's surface, said heading system comprising amagnetometer, said method comprising the steps of: obtaining an actualreading from said magnetometer at a selected magnetic heading for saidpositioned vehicle; obtaining a set of theoretical magnetic fieldproperties of the Earth at the location of the positioned vehicle, saidobtained theoretical magnetic field properties comprising theoreticalvalues for a horizontal intensity and a vertical intensity of saidmagnetic field at said location of said positioned vehicle; computing atheoretical reading for said magnetometer at said selected magneticheading of said positioned vehicle based on at least said magneticheading and said obtained theoretical values for said magnetic field atsaid location of said positioned vehicle; comparing said obtained actualmagnetometer reading for said positioned vehicle at said selectedmagnetic heading with said obtained theoretical magnetometer reading atsaid selected magnetic heading for providing a calibration value forsaid heading system at said selected magnetic heading for saidpositioned vehicle; and utilizing said provided calibration value atsaid magnetic heading of said vehicle for providing a universal averagegain and offset for said magnetometer; whereby said heading system isuniversally calibrated with respect to measurement errors.
 20. A headingsystem calibration method in accordance with claim 19 wherein saidvehicle is a ship positioned in water along the Earth's surface.
 21. Aheading system calibration method in accordance with claim 19 whereinsaid vehicle is selectively positioned to at least four differentmagnetic headings for said vehicle, said method further comprisingrepeating said obtaining, computing, and comparing steps at each of saidfour different selected magnetic headings for said vehicle for obtainingcalibration values for said heading system at each of said fourdifferent selected magnetic headings for said vehicle, said utilizingstep further comprising averaging all of said four provided calibrationvalues for providing said universal average gain and offset for saidmagnetometer.
 22. A heading system calibration method in accordance withclaim 21 wherein said four different selected magnetic headings for saidvehicle comprise the North, South, East, and West magnetic headings forsaid vehicle during said calibration.
 23. A heading system calibrationmethod in accordance with claim 19 further comprising repositioning saidvehicle to a different selected magnetic heading, repeating each of saidobtaining, computing, and comparing steps at said different selectedmagnetic heading for said repositioned vehicle for providing anothercalibration value for said heading system at said different selectedmagnetic heading for said repositioned vehicle, said different selectedmagnetic headings for said vehicle being selected from the groupconsisting of North, South, East, and West magnetic headings for saidvehicle during said calibration, said utilizing step further comprisingaveraging said provided calibration values at said magnetic headings ofsaid positioned aircraft for providing said universal average gain andoffset for said magnetometer.